Conventionally, a developing device using a two-component developer composed of toner and a carrier is used. In such a developing device, the toner is consumed by the developing operation, though the carrier remains in the developing device without being consumed. The carrier stirred together with the toner in the developing device, as the stirring frequency increases, causes a situation that the resin coat layer on the surface is separated or the toner is adhered onto the surface, thereby causes deterioration. As a result, the charging characteristic of the developer is lowered slowly.
Therefore, there is a developing device available, separately from feed of toner consumed by development, for feeding a carrier little by little into the developing device, thereby suppressing a lowering of the charging characteristic.
This developing device permits the developer becoming excessive due to feed of a carrier to overflow and discharge from the developer discharging port for developer discharge installed at a predetermined position on the wall of the developer bath. By successive repetition of such feed and discharge, the deteriorated developer in the developing tank is replaced with toner and a carrier which are fed newly.
By doing this, the developer quantity in the developing tank is kept constant and the charging characteristic of the developer is maintained and the copied image quality is prevented from lowering, thus the image formation can be performed satisfactorily.
However, in this overflow system, the supply of the developer to the developing device and the discharge of the developer from the developing device are not interlocked with each other. Therefore, the quantity of the developer in the developing device depends on an environmental change. For example, when the developer is increased in quantity, the drive torque of the developing device becomes high and there are possibilities of damage of the gears of the developing device. When the developer is decreased in quantity, in correspondence to the conveying screw pitch of the developing device, image irregularities are caused easily.
To solve the aforementioned problems, in Japanese Patent Application Publication No. 10-90991, an art of detecting accurately the supply quantity and discharge quantity of a developer, controlling the developer supply and discharge highly precisely, thereby maintaining the developer quantity constant is disclosed.
In the apparatus disclosed in Japanese Patent Application Publication No. 10-90991, a problem arises that in order to detect accurately the toner feed quantity and toner discharge quantity, the toner consumption quantity detecting means and exhausting developer quantity detecting means are used, thus the necessary parts of the apparatus are increased in number and the structure is complicated in correspondence to it and the cost thereof is increased.